Offshore production facilities use pipelines to transport natural gas extracted from deep sea gas fields or gas-laden oil-rich fields to processing facilities located on the nearest shore. These onshore facilities chill the natural gas, which turns the gas into a liquid and shrinks its volume by about 600 times. This chilling is accomplished by a liquefied natural gas (“LNG”) refrigeration train (or, simply, an LNG train). The LNG produced by the train is then loaded onto ships for transportation to customers. Alternatively, in a very different and new processing approach, the natural gas is processed into LNG at sea on board a ship. Such ships are known as floating liquefied natural gas (“FLNG”) facilities.
Regardless of whether the gas is being liquefied at an onshore facility or at sea, some pretreatment of the incoming natural gas stream is required to remove contaminants, such as water, carbon dioxide, hydrogen sulfide, and mercury, so that the gas stream being processed by the LNG train is about 99 percent pure gas. This pretreatment is usually accomplished by way of an amine unit, which removes hydrogen sulfide and carbon dioxide, and a dehydration unit, which removes water. The pretreatment system then delivers the substantially pure natural gas stream to the LNG train at a temperature of about 37° C. to 49° C. (100° F. to 120° F.).
Energy consumption is a key cost driver in all LNG trains. In particular, the natural gas stream must be cooled or refrigerated to about −160° C. (−256° F.) to make LNG. Because the refrigeration process consumes more energy than any other process in the LNG train, energy optimization and conservation are key design considerations. The volumes of gas being treated are typically very large, so even small decreases in the temperature of the incoming gas stream to the LNG train may translate into significant energy and cost savings.
A FLNG train has additional challenges in this regard because of its location offshore and the limited amount of space available aboard ship. For example, the footprint of an FLNG train may be about one-fourth the size of an onshore LNG processing facility with equivalent capacity. A need exists for a pretreatment system that has a smaller footprint than conventional pretreatment systems and that can deliver the incoming gas stream to the LNG train at a lower temperature, thereby reducing the amount of energy required for the refrigeration process.